If it's new, unfired and in the box, it is probably a recent production one I am guessing. In that case, the 80% rule is a good place to start. That is the pretty standard rule that any "like new" gun still in production is worth about 80% of the current retail/street price.
I see new ones at $500/$525, so I would guess $400. This is an estimate; you can check gunbroker and places to see what used ones are actually doing, for example.